This application requests funds for purchase of a two-photon microscopy system with imaging and uncaging capabilities that will constitute a shared facility in the Departments of Neurosciences and Radiology at the University of California, San Diego. Two-photon microscopy is becoming an important tool for biomedical research, providing the ability to image intact brain tissue to depths of up to >500 microns, with sub-micron resolution. We have assembled an outstanding broadly based group of Users from multiple departments at UCSD with a major focus on pre-clinical studies in animal models of neurological conditions and cancer. We expect the User base to grow, as new projects get funded, and as we attract additional investigators. The proposed research involves neurovascular coupling, stroke, Parkinson's and Alzheimer's diseases, nerve regeneration, injury repair, cortical communication and developmental neuroscience. These research projects require deep penetration of light in tissue, minimization of photodamage, ability to image and co-localize multiple fluorescent indicators, sub-cellular photoactivation and capability of simultaneous electrophysiological recordings. All these requirements are addressed by the proposed state-of-the-art two-photon microscopy multi-user facility. The requested system was chosen because of its high image quality, high acquisition speed, easy calibration of the two-photon imaging and uncaging optical pathways and compatibility with electrophysiology components. The system allows versatile imaging of the structure and function under in vivo, in vitro and ex vivo conditions, supports two-photon photoactivation of caged compounds and Fluorescent Lifetime Imaging Microscopy (FLIM). Administratively, the proposed instrument will be part of The Center for fMRI at UCSD. The Center provides comprehensive imaging facilities for users in the greater San Diego, with established infrastructure for resource administration, recharge mechanisms, and user training. The proposed two-photon instrument is complementary to the existing neuroimaging modalities at the Center such as MEG, PET and fMRI, and will provide an essential and central component of the UCSD Multimodality Imaging effort. Specifically, combining fMRI, PET and in vivo two-photon microscopy will facilitate bridging the gap between non-invasive human and single-cell animal characterization of disease-related changes. Studying of animal models of human diseases is a major ongoing effort at UCSD School of Medicine. In summary, the proposed cutting-edge two-photon facility will provide NIH-funded investigators at UCSD with an essential tool to study the intricate pathways involved in brain activation, features of disease progression, disease diagnosis and treatment. These studies will impact our understanding of normal and pathological brain function, opening new avenues for treatment and prevention of human disease. The proposed facility is also expected to play a significant role in the training and teaching efforts at UCSD.